Waiting for a whole-cloth Servo-based browser to be totally competitive would be not learning from the not-so-distant past, where Netscape 5 was canceled in favor of the "correct" option of shipping the totally-rewritten Mozilla Suite. Netscape 4 withered away in the market, and it took additional years for the Mozilla browser to be ready and Firefox for it to find its footing.
Making these changes to Firefox benefits users now, and deprecating XPCOM-based extensions in favor of Web Extensions helps pave the way to supporting other rendering engines, which will never support XPCOM (like Servo).
In the meantime Rust is finding its way into Firefox, so code and lessons being learned in Servo (and elsewhere) can find their way to users sooner.
I'm wondering, what kind of security benefits you had in mind?
(someone correct me if I'm wrong)
In my opinion, Mozilla shouldn't have even tried to implement Electrolysis in Firefox and shouldn't have announced a switch to WebExtensions in it either. Instead, it should've done what Microsoft did and create a new browser in Rust from scratch, with a much cleaner, easier to maintain codebase that would also be much more secure in the long term.
I don't know if Mozilla's funding would even allow for that, but I think that would've been the better strategy. The putting lipstick on a pig usually doesn't work, and it usually pisses off those who preferred the way the pig looked before, too, causing said platform to lose both old and potentially new users.